A diode is a semiconductor device having a non-linear voltage-current relation. Diodes are important solid-state devices and have many electronic applications. The tunnel diode is a variety of diode with the unusual characteristic of negative differential resistance. Negative differential resistance is a voltage-current behavior in which, over certain voltage ranges, increasing the voltage applied across the diode leads to decreased current carried through the diode. For certain voltage ranges, however, current flows relatively freely through the diode. In contrast, for most devices, increasing the voltage applied across the diode, within operating parameters, leads to increasing current regardless of the voltage range. Tunnel diodes, in general, have a number of applications, including high frequency oscillator circuits and high frequency electronic switches.
One type of tunnel diode is the double barrier tunnel diode, which generally includes a quantum well with thin barrier layers on either side. This structure is known as a double barrier structure and typically lies between two injection layers. The double barrier structure serves as an energy barrier to the flow of electrons that can be overcome only under certain conditions. Fulfillment of these conditions results in a negative differential resistance characteristic of interest over a range of external applied bias voltages. Electrons are injected into the double barrier structure from the conduction band of one of the injection layers under an internal electric field produced by the applied external bias voltage. The applied voltage increases the energy of the injected electrons such that they satisfy the resonant tunneling condition of the quantum barrier. Under such voltage conditions, the resonance condition is satisfied and an incoming electron has the same energy as an energy state of the quantum well. This condition enables electrons to tunnel through double barrier structure. As the bias voltage is increased further, the energy levels no longer match the energy state of the quantum well and the resonance condition is no longer satisfied. At this point the current decreases, resulting in the negative differential resistance effect.
Of particular interest are quantum well devices having current voltage characteristics including multiple negative differential resistance regions. Using traditional methods to achieve multiple negative differential resistance regions required complex circuitry. Therefore, development of a simpler, integrated circuit exhibiting multiple negative resistance regions was desirable. Such multiple regions may be obtained from a plurality of resonant states of a quantum well or from stacking several quantum wells together. However, the resulting devices typically require much higher voltages corresponding to excited states as compared to the resonant voltage of a single quantum well.
As a solution to this problem, and in furtherance of the continuing trend towards miniaturization and increased functional density in electronic devices, much attention has been directed toward resonant-tunneling devices as characterized by operation involving a particular carrier energy coinciding with a particular quantized energy level in a potential well. Extensive literature has been developed surrounding both the theoretical and practical device aspects as surveyed, e.g., by
F. Capasso et al., "Resonant Tunneling Through Phenomena . . . in Superlattices, and Their Device Applications", IEEE Journal of Quantum Electronics, Vol. QE-22 (1986), pp. 1853-1869. PA1 Also, noting that resonant-tunneling devices may be made as diodes and as transistors; see, e.g., PA1 E. R. Brown et al., "Millimeter-band Oscillations Based on Resonant Tunneling in a Double-barrier Diode at Room Temperature", Applied Physics Letters, Vol. 50 (1987), pp. 83-85; PA1 H. Toyoshima et al., "New Resonant Tunneling Diode with a Deep Quantum Well", Japanese Journal of Applied Physics, Vol. 25 (1986), pp. L786-788; PA1 H. Morkoc et al., "Observation of a Negative Differential Resistance Due to Tunneling through a Single Barrier into a Quantum Well", Applied Physics Letters, Vol. 49 (1986), pp. 70-72; PA1 F. Capasso et al., "Resonant Tunneling Transistor with Quantum Well Base and High-energy Injection: A New Negative Differential Resistance Device", Journal of Applied Physics, Vol. 58 (1985), pp. 1366-1368; PA1 N. Yokoyama et al., "A new Functional, Resonant-Tunneling Hot Electron Transistor (RHET)", Japanese Journal of Applied Physics, Vol. 24 (1985), pp. L853-L854; PA1 F. Capasso et al., "Quantum-well Resonant Tunneling Bipolar Transistor Operating at Room Temperature", IEEE Electron Device Letters, Vol. EDL-7 (1986), pp. 573-575; PA1 T. Futatsugi et al., "A Resonant-Tunneling Bipolar Transistor (RBT): A Proposal and Demonstration for New Functional Devices with High Current Gains", Technical Digest of the 1986 International Electron Devices Meeting, pp. 286-289; PA1 T. K. Woodward et al., "Experimental Realization of a Resonant Tunneling Transistor", Applied Physics Letters, Vol. 50 (1987), pp. 451-453; PA1 B. Vinter et al., "Tunneling Transfer Field-effect Transistor: A Negative Transconductance Device", Applied Physics Letters, Vol. 50 (1987), pp. 410-412; PA1 A. R. Bonnefoi et al., "Inverted Base-collector Tunnel Transistors", Applied Physics Letters, Vol. 47 (1985), pp. 888-890; PA1 S. Luryi et al., "Resonant Tunneling of Two-dimensional Electrons through a Quantum Wire: A Negative Transconductance Device", Applied Physics Letters, Vol. 47 (1985), pp. 1347-1693; and PA1 S. Luryi et al., "Charge Injection Transistor Based on Real-Space Hot-Electron Transfer", IEEE Transactions on Electron Devices, Vol. ED-31 (1984), pp. 832-839.
Of particular interest are integrated devices having asymmetric current-voltage characteristics and multistable states. An example of previous methods for obtaining multiple stable solutions by putting two resonant-tunneling diodes in parallel is that of Capasso et al., U.S. Pat. No. 4,902,912 in which the two resonant-tunneling diodes are separated by a resistance consisting of a low doped region between the resonant-tunneling diodes. Although the current out of the contact exhibits double peaks and multiple stable states for certain voltage differences, the required use of multiple voltage sources increases the complexity of the device. Additionally, the device is not easily adjustable or controllable. An attempt to improve on the concept developed by Capasso et al. was made by J. Soderstrom and T. Anderson, as discussed in Electron Device Letters, Vol. 9, No. 5, May 1988. Soderstrom and Anderson introduced two resistors in series with the resonant-tunneling diodes. By choosing the resistors properly, they caused the circuit to produce a double peak curve. However, this concept has the disadvantage that the resistors introduce hysteresis, which causes the presence of different peak voltages for forward and backward voltage sweeps, and also decreases the speed of the device due to the RC time constant.
Accordingly, it is an object of the present invention to provide an integrated circuit including an oppositely connected resonant tunneling diode pair with an asymmetric current-voltage characteristic which exhibits multistable states and can be tailored for multistable solutions. It is another object of the present invention to provide apparatus in accordance with the invention, which provide examples of its best-mode usage.